This invention relates to a novel method and optical system which has inherent advantages over existing monochromators and scanning spectrometers employing diffraction gratings. The invention is particularly well adapted to use at grazing incidence.
Diffraction gratings have been widely used in spectroscopic analysis for over a century. In many experimental instances, it is necessary to change the wavelength which is diffracted to a fixed location. When such location presents a spatial filter (e.g. a slit or pinhole), the resulting monochromator is provided with means capable of scanning any wavelength from a limited spectral continuum through the exit. If a detector is placed at the grating focus in place of a slit, the resulting spectrometer often retains the need to tune the wavelength diffracted to the detector center.
In all such monochromators and spectrometers to date, the scanning (or tuning) of wavelength has been accomplished by movements of the grating (or separate auxiliary optics) within the dispersion plane which is normal to the grating grooves. As illustrated in FIG. 1, the only grating rotation motion employed in the prior art has been about an axis parallel to the grating grooves. The only other grating motions in the prior art have been translations within the plane normal to the grooves, and combinations of such rotation and translations. This constraint has been honored by all grating instruments, whether they be used at normal incidence or grazing incidence, for in-plane or off-plane diffraction, in reflection or transmission, and even when the groove spacings have been varied.
Unfortunately, such movements within the dispersion plane of the grooves alter the grating focusing. Often, this means the introduction of auxiliary mirrors and complicated scanning motions for the optics and possibly the slits to maintain a well focused image at all wavelengths. In addition, for all instruments except those which adhere to the Rowland circle [H. A. Rowland, Phil. Mag. vol 16 (1883), p. 197], the numerical aperture exiting a given size grating varies significantly with the selected wavelength as the grating is rotated. This imposes a limitation on the range in wavelengths which can be scanned with a single grating, and requires exit baffling to reject the rays which do not match the fixed aperture of most targets to which such radiation is directed.
The magnitude of all these effects is highest when the grating is operated at grazing angles to the incident radiation. Such illumination is required to efficiently reflect electromagnetic radiation having wavelengths shorter than approximately 1000 .ANG.. Therefore spectroscopic instruments of the prior art exhibit a number of undesirable and cumbersome characteristics which compromise experiments performed in the vacuum ultraviolet (&lt;1000 .ANG.) and soft x-ray (&lt;100 .ANG.) regions of the spectrum.
For example, moving slits are usually required in monochromators based on the Rowland circle, such as the commercial design of McPherson in U.S. Pat. No. 3,211,049, the "Vodar" monochromator of Salle et al in Compt. Rend. vol. 230 (1950), p. 380, the adjustable monochromator of Tondello et al in U.S. Pat. No. 4,254,335, the "Grasshopper" monochromator of Brown et al in U.S. Pat. No. 4,398,823, and the monochromator of Kaffka in U.S. Pat. No. 4,605,306. Even the varied-space grating in the non-Rowland monochromator of Turner et al in U.S. Pat. No. 4,027,975 requires use ov moving slits. Auxiliary mirrors in addition to wavelength-dependent apertures are characteristic of plane grating monochromators, such as those of Itou et al, Appl. Opt. vol. 28 (1989), p. 146, Hettrick et al in U.S. Pat. No. 4,776,696 ("HIREFS"), Pouey in U.S. Pat. No. 4,241,999 ("Monograph"), Petersen in U.S. Pat. No. 4,553,253 ("SX-700"), Kabler et al in U.S. Pat. No. 4,462,689, Dietrich et al in Rev. Sci. Instrum. vol. 43 (1972), p. 434 ("Flipper"), and Werner et al in Appl. Opt. vol. 20 (1984), p. 23. Spectral images are not in focus for fixed-slit off-Rowland concave grating monochromators, such as the constant angle of incidence mounting of Axelrod in U.S. Pat. No. 3,495,909, the Johnson-Onaka monochromator of Onaka, Sci. Light vol. 7 (1958), p. 23, the toroidal grating monochromator ("TGM") of Madden et al, J. Opt. Soc. Am. vol. 62 (1972), p. 722, and the high-throughput monochromator ("HTM") of Hettrick et al, Appl. Opt. vol. 25 (1986), p. 4228. To maintain even an appropriate focus with fixed slits as such an off-Rowland concave grating is rotated, an auxiliary mirror and a translation of the entire optical system is required in the monochromator of Ishiguro et al in Rev. Sci. Instrum. vol. 60 (1989), p. 2105.
A couple self-focusing monochromator designs based on the use of varied-space gratings have avoided most, but not all of the above problems. Aspnes in U.S. Pat. No. 4,492,466 describes a clever cylindrical grating monochromator ("CGM") design in which the grating simply translates along its symmetry axis (within the dispersion plane) to select wavelength. This essentially maintains a stigmatic focus at a fixed exit pinhole without any auxiliary mirrors. However, the translation is enormous, leading to use of only a small fraction of the full grating aperture at any particular wavelength. Recently, Hettrick in U.S. Pat. No. 4,991,934 describes an In-Focus Monochromator ("IFM") whose grating scans wavelength by rotation about a fixed axis combined with a small linear translation along its surface. However, the grating rotation still leads to a wavelength-dependent output aperture, typically being approximately a factor of two over the scanned spectrum. If no such variation is tolerable (e.g. a given target aperture must be matched at all wavelengths), then the baffled exit aperture results in a factor of two loss in throughput at an extreme wavelength.
In the case of a flat-field spectrometer, designs employing a concave varied-space grating, such as that of Fonck et al in Appl. Opt. vol. 21 (1982), p. 2115 and Kita et al in Appl. Opt. vol. 22 (1983), p. 512, a rotation of the grating about its grooves results in significant changes in the focal length. The result is that such spectrometers are not tuneable in wavelength striking a fixed detector position unless additional auxiliary mirrors are employed.
Grating monochromators and spectrometers which change wavelength over a wide region at high efficiency without altering the spectral focus, the slit positions, or the beam aperture, and do not require auxiliary mirrors or complex scanning motions, would be a highly useful advance in the field of optics.